There are many different types of technologies employed in calculating the location of mobile stations in wireless networks with various levels of success and accuracy. Assisted-GPS (A-GPS) is a positioning technology that is presently used for locating mobile stations in wireless networks. An A-GPS server provides assistance data to the mobile station in order for it to have a low Time to First Fix (TTFF), to permit weak signal acquisition, and to optimize mobile station battery use. A-GPS is used as a location technology in isolation or hybridized with other positioning technologies that provide range-like measurements.
An A-GPS server provides data to a wireless mobile station that is specific to the approximate location of a mobile station. The assistance data helps the mobile station lock onto satellites quickly, and potentially allows the handset to lock onto weak signals. The mobile station then performs the position calculation or optionally returns the measured code phases and potentially carrier phases to the server to do the calculation. The A-GPS server can make use of additional information such as round-trip timing measurements from a cellular base station to the mobile station in order to calculate a location where it may otherwise not be possible, for example when there are not enough GPS satellites visible.
Advances in satellite-based global positioning system (GPS), timing advance (TA), and terrestrial-based enhanced observed time difference (E-OTD) position fixing technology enable a precise determination of the geographic position (e.g., latitude and longitude) of a mobile station subscriber. As geographic location services are deployed within wireless communications networks, such positional information may be stored in network elements and delivered to nodes in the network using signaling messages. Such information may be stored in SMLCs (Serving Mobile Location Centers), SASs (Stand-Alone SMLCs), PDEs (Position Determining Entities), SLPs (Secure User Plane Location Platforms) and special purpose mobile subscriber location databases.
One example of a special purpose mobile subscriber location database is the SMLC proposed by the 3rd Generation Partnership Project (3GPP). In particular, 3GPP has defined a signaling protocol for communicating mobile subscriber positional information to and from an SMLC. This signaling protocol is referred to as the Radio Resource LCS (Location Services) protocol, denoted RRLP, and defines signaling messages communicated between a mobile station and an SMLC related to a mobile subscriber's location. A detailed description of the RRLP protocol is found in 3GPP TS 44.031 v7.2.0 (2005-11) 3rd Generation Partnership Project; Technical Specification Group GSM Edge Radio Access Network; Location Services (LCS); Mobile Station (MS)—Serving Mobile Location Center (SMLC) Radio Resource LCS Protocol (RRLP) (Release 7).
In addition to the United States Global Positioning System (GPS), other Satellite Positioning Systems (SPS), such as the Russian GLONASS system or the proposed European Galileo System, or the proposed Chinese Compass/Beidou System may also be used for position location of a mobile station. However, each of the systems operates according to different specifications. In particular, each of the systems uses its own specific system time.
With the launch of Galileo, the number of visible navigation satellites is basically doubled at a combined GPS/Galileo receiver, or trebled at a combined GPS/GLONASS/Galileo receiver, which in general improves service availability and accuracy. The additional satellites provide redundancy and can be used to e.g., eliminate poor quality measurements while maintaining sufficient Geometric Dilution of Precision (GDOP). In certain critical navigation scenarios, an e.g., combined GPS/Galileo receiver may be able to obtain a position fix where either GPS or Galileo alone does not provide enough satellite measurements to obtain a successful navigation solution.
GPS (Global Positioning System) and Galileo are independent navigation systems and therefore, each system uses its own navigation time reference. GPS System Time is steered to universal temps coordine' (UTC) (universal coordinated time) as maintained by the US Naval Observatory, referred to as UTC (USNO). GPS System Time is specified to be kept within 1 microsecond modulo 1 second of UTC (USNO).
Galileo System Time is expected to be steered to International Atomic Time (TAI) and is specified to be kept within 50 ns of TAI. Apart form a potential integer number of seconds, the offset between GPS and Galileo System Time is expected to be in the order of tens of nanoseconds.
The offset between GPS and Galileo System time will be included in the Galileo broadcast navigation message as well as in the future GPS broadcast navigation message, and is referred to as GPS-Galileo Time Offset, or more generally, to GPS-GNSS Time Offset (GGTO). For GPS, the GGTO is already specified in Message Type 35 of the December 2004 version of IS-GPS-200, Revision D, which may include the GPS-Galileo GGTO as well as the GPS-GLONASS GGTO.
For example, a combined GPS/Galileo receiver that uses GPS and Galileo pseudo-range measurements in the navigation solution can perform according to the three following options.                1. Ignore the GPS-Galileo time offset in the position calculation.        2. Use an additional satellite signal measurement to obtain the GGTO as part of the navigation solution.        3. Use the GGTO either available from decoding the navigation message or via assistance data provided by the cellular network.        
The first option would likely result in a biased navigation solution. The amount of bias depends on the actual GGTO, and may be acceptable in certain scenarios, but of course, is less desirable.
The second option would require at least one additional satellite measurement in the navigation solution, which may not always be available in certain critical scenarios (e.g., indoors or urban canyons). At least five satellite measurements would be required to solve for three-dimensional position, receiver time bias, and GGTO, but more than five would be desirable to improve accuracy.
The third option does not require an additional satellite measurement and is the preferred approach in environments with limited satellite visibility (e.g., indoors or urban canyons). The GGTO may be obtained by decoding the satellite navigation message (which however, requires sufficient satellite signal strength and takes a relative long time; i.e., increases the TTFF), or may be provided in the assistance data message (which however, requires modifications to standardized location protocols). The user receiver has to take the GGTO into account when combining pseudo-ranges for GPS and Galileo satellites. Either the GPS or Galileo observations have to be corrected for the GGTO before applying the navigation solution.
However, handling the various different system times and time offsets in the navigation solution requires substantial modifications to existing user receivers, and would undoubtedly raise the costs associated with production of next generation user receivers having this capability.
Accordingly, there is a need for a communication system, including a global navigation satellite system (GNSS), which can determine a position location for a mobile station based on satellite signals sent from two or more satellite systems, rather than just one satellite system, to provide further efficiencies and advantages for position location without the need for handling various GNSS system times at the mobile receiver and without specifying a completely new location protocol for assistance and mobile measurement data transfer.
Moreover, there is a need in the art for a communications system, method and/or apparatus that is adapted to calculate and correct for the GGTO without adding significant upgrade or production costs to existing and future user receivers.